28.已知集合A={x∣x2−5x⩽aA=\{x\vert x^{2}-5x\leqslant aA={x∣x2−5x⩽a,a∈R}a\in R\}a∈R},集合B={x∣log2x⩽1}B=\{x\vert \log _{2}x\leqslant 1\}B={x∣log2x⩽1}. (1)当a=−4a=-4a=−4时,求A⋂BA\bigcap BA⋂B; (2)若A⋃B=AA\bigcup B=AA⋃B=A,求实数aaa的取值范围.